The effects of functional group reagents, affinity labeling reagents, and isotopic probes on the catalytic activity of catechol-O-methyltransferase (COMT) are being investigated. The objectives of this study are to determine the mechanism of catalysis of COMT and the nature of the amino acid residues present at its active site. Attempts are also being made to identify reversible inhibitors of COMT, which might be effective in vivo as experimental tools or chemotherapeutic agents. Derivatives of 3, 4-dimethoxy-5-hydroxy phenylethylamine are being synthesized and evaluated in vitro and in vivo for their abilities to inihibit COMT. Comparisons are being made between the affinity of these inhibitors for the active site of COMT and their abilities to serve as substrates for the metabolic enzyme phenol-sulfotransferase.